Joining Methods and Fastenings
Joining Methods and Fastenings tests permanent and temporary joints, correct fastener selection, and safe soldering, brazing and riveting.
Permanent joints
Welding fuses the parent metal. Brazing and soldering use a filler with a lower melting point. Soft soldering, with tin-lead or lead-free solder, suits electrical and sheet-metal joints; it needs clean surfaces and flux. Brazing is stronger and works at a higher temperature. Rivets form a permanent joint: snap-head and countersunk solid rivets, and blind pop rivets for one-sided access. Adhesives suit dissimilar materials and thin sheet but need clean, roughened surfaces.
Temporary fastenings
Bolts and nuts, machine screws, self-tapping screws, set screws, keys and pins allow dismantling. A bolt passes through a clearance hole and is tightened with a nut; a machine screw threads into a tapped hole. Match thread type and pitch, metric coarse or fine, or imperial, and use washers to spread load.
Worked example: riveted lap joint
Drill clearance holes in both sheets, deburr, align and clamp. Insert a solid rivet, place a dolly behind the head, and form the second head with a ball-peen hammer or rivet snap. Space rivets about three diameters apart and keep edges at least two diameters from the rivet centre. Reasoning: correct spacing spreads load and prevents tearing, and deburring lets the sheets pull tight.
Traps
Soldering dirty or greasy metal, using too much flux or overheating a joint gives a weak joint. Do not use a pop rivet where vibration or high load demands a solid rivet or bolt. Mixing metric and imperial threads damages both. Welding without a helmet and screens is unsafe and loses marks.